Programmable body implanatable stimulators are known to the prior art. Their programming has been variously accomplished as through the use of a magnetic field operating on an implanted reed switch, the use of radio frequency energy transmitted to an antenna within the implanted unit and through the use of a Keith needle. A magnetic field alone has the obvious disadvantage of being easily duplicated by extraneous fields to result in an undesired programming. A Keith needle requires a penetration of the body. The radio frequency energy approach, as implemented in the prior art, has also had its drawbacks.
One system which combines a magnetic field with a radio frequency signal for the programming of a body implantable stimulator is disclosed in U. S. Pat. No. 4,066,086 issued Jan. 3, 1978, to Clifton A. Alferness et al. for PROGRAMMABLE BODY STIMULATOR, which is hereby incorporated by reference. Another programmable unit is disclosed in application Ser. No. 957,813 now U.S. Pat. No. 4,275,737 filed Nov. 6, 1978, in the name of Jerome T. Hartlaub et al. for DEMAND CARDIAC PACEMAKER HAVING REDUCED POLARITY DISPARITY, which is commonly owned with the present invention and which is also incorporated herein by reference.
Many prior art implantable stimulators have been formed by molding the components, including mechanical and electrical connections for the lead, in a matrix of encapsulating material which supports the components and shields them from the body environment. Typically, the encapsulating material is an epoxy. However, it is generally recognized that an enclosed and hermetically sealed unit is more reliable as a result of the known and controlled environment provided by the hermetic seal. For this reason, many recent signal generator designs include a rigid enclosure formed of a plurality of preformed members which are typically welded together to complete the enclosure. In the context of a programmable unit, however, absorption, reflection and attenuation of radio frequency energy by the metal enclosure complicates the programming operation. These programming problems are further compounded by power transmission limits established by the FCC and the trend towards smaller and smaller units.